media: dt-bindings: media: nxp,imx7-mipi-csi2: Expand descriptions

Expand the description of the binding itself and of individual
properties to include additional information that may not be immediately
appartent from reading the reference manual. Drop the last sentence of
the phy-supply description that refers to the driver's implementation.

While at it, fix the capitalization of MIPI CSI-2 in the title.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>